Gastrointestinal prosthesis for restraining the rate of the gastric digestion consists of a compressible proximal member connected by a string to a distal member, to be introduced into the intestine. The proximal member has an internal space, which is opened to the gastric lumen and may include passageways for the gastric content as well as open grooves or niches disposed on its external wall. The volume of a proximal member compressed by the gastric wall cannot get smaller than a lower threshold. Anchoring the gastrointestinal prosthesis at its desired location is accomplished by means of the proximal member the geometrical shape of which conforms the geometrical shape of a portion of the gastric lumen; as well as by means of the geometrical shape of the distal member which conforms the geometrical shape of a segment of the duodenum and/or a segment of the intestine.